This invention relates to integrated circuits, and specifically to the problem of providing stabilized bias current to an amplifier used in an integrated circuit chip.
In electronic environments other than integrated circuits (IC), suitable resistance components are normally used to provide stabilized bias current, i.e., a current source having minimum amplitude variation (ripple). However, resistance components are not suitable for most integrated circuit uses. One reason is that they require more "real estate" than is likely to be available. Also, they tend to have undesired variability of resistance value from component to component; and they tend to vary in resistance value with temperature changes. In IC circuits, desired equivalent resistance values may be obtained using switched capacitor circuitry. Such circuitry may be used, for example, as the feedback resistance of an IC operational amplifier, as shown in U.S. Application Ser. No. 558,009, filed 12/5/83 assigned to the assignee of this application.
In fact, the problem addressed by the present application was encountered in working with the circuitry shown in FIG. 2 of Application Ser. No. 558,009. In that figure, two transistors are shown which provide separate constant bias current sources, respectively, for a differential amplifier and for a source follower output transistor. However, although the present invention was conceived in response to the need for reliable constant bias current sources in the environment of the IC of Application Ser. No. 558,009, it has much broader potential uses.
The switched capacitance technique for providing a resistance-equivalent, which was discussed above, is not suitable for use as the resistance-equivalent in a constant current source. This is true because of the large current transients, or pulsations, that occur during the switching cycle of the switched capacitance network.